The present invention relates to a two-battery system and a method for controlling the two-battery system.
Such a two-battery system having a vehicle electrical-system battery and a starter battery, which are interconnected when the generator is running, is described in German Published Patent Application No. 38 41 769. In the case in which a small amount of current is being supplied by the generator, this has the disadvantage of the electrical-system battery being charged at the expense of the starter battery, due to charge equalization, when the state of charge of the electrical-system battery is worse than that of the starter battery. In the conventional vehicle electrical system, a switch is also installed between the electrical-system battery having the electrical-system loads, and the remaining part of the vehicle electrical system, the switch being opened in response to the generator shutting down, as soon as the starter-battery voltage falls below a permissible limiting value. This measure only protects the starter battery from being completely discharged when the generator is stopped, and it also allows the starter battery to be partially discharged. This poor initial condition increases the load on the starter battery at the beginning of the trip, when the charge of the two batteries is equalized at the expense of the starter battery. In the extreme case, this can result in the complete discharge or destruction of the starter battery.
A two-battery system is described in German Published Patent Application No. 40 28 242, where the connection between the starter battery and the electrical-system battery can be opened, when the electrical-system battery would otherwise be charged by the starter battery. The two batteries are connected to each other, when the voltage of the electrical-system battery is greater than the starter-battery voltage. If the voltage of the electrical-system battery is less than that of the starter battery, then the two batteries are disconnected from each other. The separation can be performed in various ways, e.g. by disconnecting the starter battery from the rest of the vehicle electrical system. In this manner, the starter battery is not loaded by a discharged electrical-system battery, and, in the working mode, the generator can power the most important consumers and simultaneously charge the electrical-system battery. The two batteries can also be disconnected from the rest of the vehicle electrical system by switching off the electrical-system battery. The starter battery can then be charged, when the generator and the starter battery are connected.
When space is limited, the positioning of the starter battery in the motor vehicle is a general problem in two-battery systems. In order to reduce the fire hazard from electrical short-circuiting during accidents, the starter line, which electrically connects the starter to the starter battery in a permanent manner, must be laid through impact-resistant zones in the motor vehicle. This also applies to the starter line in single-battery systems. As an alternative, pyrotechnic battery terminals may be used, which are irreversibly severed from the batteries in an explosive manner prior to or during an accident, and thus interrupt the flow of electric current. In this context, the firing control of the pyrotechnic terminals is mostly performed by a precrash sensory system, which, e.g., evaluates the signals of an airbag control unit. In addition to their high cost, the complex firing control of the pyrotechnic terminals is disadvantageous, since inadvertent triggering must be prevented or the motor vehicle can no longer be started. The irreversibility is generally disadvantageous, since the terminals must be replaced after each instance of triggering.
Therefore, it is an object of the present invention to provide a two-battery system and a method for controlling the system, which permit simple starter management and charge management for the starter battery. It is another object of the present invention to provide a simple and cost-effective reduction of the fire hazard.
The above and other beneficial objects are achieved by providing a two-battery system and a method as described herein.
To this end, a first, electrically fully-blocking switch element is arranged between the start-relevant load circuits and the starter battery, and a second, electrically fully-blocking switch element is arranged between the electrical-system battery and the start-relevant consumers. An element that blocks current from flowing from the starter battery to the electrical-system battery and the electrical-system consumers are arranged between the starter battery and the electrical-system battery. Using the switch elements, the start-relevant consumers may be positioned on alternate sides of the starter battery or the electrical-system battery, while the element prevents the starter battery from being discharged by a discharged electrical-system battery. Electrically fully-blocking means that no significant current may flow through the switch element in the open or blocked state. In particular, this includes all indirect-coupled, direct-connected or direct connecting switches such as relays. However, simple power MOSFET""s may not be suited for this purpose, since they may not be completely blocked, due to a technologically necessary, parasitic diode. However, two power MOSFET""s connected back-to-back may principally be used as a switch element, since the reverse diode currents are negligible.
The element between the starter battery and the electrical-system battery may take the form of an automatically controlled, self-regulating, diode or a controllable switch or relay.
When at least one electronic pole terminal, by which the starter line may be switched off-circuit, is positioned between the starter and the starter battery in a two-battery system, the starter line, which is normally off-circuit, may be arbitrarily positioned in the motor vehicle, without representing a fire hazard. This increases the degree of freedom in the wiring, since space restrictions are largely eliminated. To this end, the electric pole terminal is only switched through in the actual starting phase and is otherwise operated in the off state. In two-battery systems, depending on the further circuit diagram, it may be ensured that, if the electrical-system battery is directly connected to the starter or may be connected to the starter to support the starting operation, the electrical-system battery may also be disconnected from the starter line by an electronic pole terminal, so that the de-energized state of the starter line is ensured. In this context, the electronic pole terminals may be of the same kind or may be configured differently, depending on the circuit layout. Another advantage is that, in the standing phase, the electronic pole terminals disconnect the starter battery, along with the consumers relevant for starting, from the rest of the vehicle electrical system, so that the starter battery may provide a sufficient starting capacity over long standing phases.
When an intended start is detected, the method provides for a start signal (terminal (50)-signal) being generated, by which the electronic pole terminals are switched through in the starting phase and blocked again after the start has occurred. If the motor vehicle is configured to have a mechanical ignition lock, then the terminal (50)-signal is directly generated by turning the ignition key, and the starting procedure is immediately initiated, if the starter battery and possibly the electrical-system battery may provide sufficient voltage. If, however, the motor vehicle is connected to an electronic ignition lock, then a start-enabling control device initially checks if the start-relevant consumers receive a sufficient supply voltage. If this is the case, then the start-enabling control device generates a terminal (50)-signal, and the electronic pole terminal of the starter battery, and possibly that of the electrical-system battery, are switched through, so that the starter line, which carries a voltage, supplies the starter with a starter voltage and starts the motor or engine. The electronic pole terminals are then blocked again, and the starter line is switched off-circuit. Regardless of the configuration of the ignition lock, it may be ensured that the starter line is only connected in circuit in the immediate starting phase, and is otherwise switched off-circuit.
In a further example embodiment, an additional electronic pole terminal is arranged between the starter and the electrical-system battery, the starter being able to be started by the electrical-system battery, via the additional electronic pole terminal, without the starter voltage being reduced by voltage drops across the element. In this example embodiment, the element may take the form of a relay.
In a further example embodiment, the high-load starting consumers, which are only active during the starting phase or a short time after the start has occurred, are permanently assigned to the starter battery, so that their current does not have to flow through the relays, and the relays may therefore be dimensioned to have a lower capacity and are less expensive.
The generator may be in the form of a controllable generator, which may adjust its voltage, using a controllable regulator. In this manner, the control unit triggering the relays may instruct the generator to adjust its output voltage to the starter-battery voltage, prior to a switching operation of the relays, so that high load currents do not have to be directed in the switching operation.
The electronic pole terminals may be in the form of switchable power semiconductors, by which appropriately large amperages may be switched, using small control currents. In a further example embodiment, the electronic pole terminal may take the form of a CMOS-FET, so that an electrical power loss essentially occurs only in the switching operations. In order to generate appropriate gate voltages, the FET is configured with a charge pump, which functions up to an electrical-system voltage of, e.g. 3 V, and builds up the appropriate gate voltage. However, the closed-circuit current through the CMOS-FET may be neglected in the stationary case, so that the batteries are not loaded. In the switched-through state, such CMOS-FET""s have a contact resistance of 0.5 to 0.6 mxcexa9, so that the voltage drop at 1200 A is only 0.6 to 0.72 V. In the case of cold starting, the resistance is only approximately 0.4 mxcexa9, so that the voltage drop is 0.48 V. Therefore, almost all of the battery voltage is available to the starter.
When the starter battery is discharged, the electronic pole terminal between the starter and the starter battery prevents an external start from occurring any longer in the engine compartment. To this end, the starter battery is assigned a tap, e.g., in the form of a busbar, which may then connect an external battery to the starter battery. This tap constitutes an external-start aiding point, which may be arranged at an arbitrary location in the motor vehicle, but, e.g., in the direct vicinity of the starter battery.